1. Field of the Invention
The present invention relates to a converter for digitizing the output of a resolver (hereinafter referred to as R/D converter).
2. Description of the Related Art
A resolver is a kind of rotary transformer, and includes two stator windings and one rotor winding. The two stator windings are mechanically disposed at an angle of 90 degrees. The amplitude of a signal obtained by magnetic coupling to the stator windings becomes a function of relative position between the position of the rotor (shaft) and the stator. Thus, two kinds of output voltages (S3-S1, S4-S2), which are modulated by a sine (sine wave) and a cosine (cosine wave) of a shaft angle and are expressed by the following equations (1) and (2), are obtained. Although equation (5) indicates a signal of a resolver format, the signal of the resolver format is based on a signal [output voltages (S3-S1) and (S4-S2)] obtained from the resolver output. EQU S3-S1=E.sub.0 sin.omega.t sin.theta. (1) EQU S4-S2=E.sub.0 sin.omega.t cos.theta. (2)
Where, .theta. is an axial angle, .omega. is an angular velocity corresponding to a rotor excitation frequency (f), and E.sub.0 is an rotor excitation amplitude.
Here, using AD2S90 of Analog Device Company as an example, a conventional resolver digital converter (R/D converter 1) of a tracking system will be described.
FIG. 5 is a functional block diagram of the AD2S90.
In FIG. 5, when a transducer (rotor of the resolver) (not shown) passes through a position equivalent to a least significant bit, the output (output of a serial interface) is renewed by one LSB. Renewal of CLKOUT corresponds to increase of one LSB.
When the present word state of an up down counter 2 is made .phi., the output voltage (S3-S1) is multiplied by cos .phi. through a sine cosine multiplier 3, and the output voltage (S4-S2) is multiplied by sin.phi., whereby signals expressed by the following equations (3) and (4) are obtained. EQU E.sub.0 sin.omega.tsin.theta. cos.phi. (3) EQU E.sub.0 sin.omega.tcos.theta. sin.phi. (4)
The signals expressed by the equations (3) and (4) are subtracted through an error amplifier 4, whereby a signal expressed by the following equation (5) is obtained. EQU E.sub.0 sin.omega.t(sin.theta.cos.phi.-cos.theta.sin.phi.)=E.sub.0 sin.omega.tsin(.theta.-.phi.) (5)
Where, (.theta.-.phi.) is an angular error.
A demodulator circuit 5 and an integrator 6 are connected to an output side of the error amplifier 4, and a voltage controlled oscillator (VCO) 7 is connected to the demodulator circuit 5 and the integrator 6. Then, the demodulator circuit 5, the integrator 6, and the VCO 7 form a close loop, which operates in such a manner that sin(.theta.-.phi.) [see the equation (5)] is made zero. When this operation is realized, the word state .phi. of the up and down counter 2 connected to the sine cosine multiplier 3 becomes equal to the resolver shaft angle .theta..
However, in the foregoing R/D converter 1 of the tracking system shown in FIG. 5, attention has not been paid in the following points.
(a) Since it is a kind of PLL control, response speed is low. Particularly, as resolution becomes high, the response speed becomes low.
(b) It is necessary to compensate temperature drift of the analog operating circuit (the sine cosine multiplier 3, the demodulator circuit 5 and the like), so that its IC circuit becomes complicated and expensive.
(c) If an interface cable between the resolver (not shown) and the R/D converter 1 becomes long, a phase becomes large because of delay between the exciting sine wave and the sine-wave output and cosine-wave output of the resolver, and an angular error becomes large.